Organic coatings are generally used to protect the surface of materials from incidental damage, abrasion, chemical attack and from environmental or in-service degradation. Organic coatings are also used to enhance the aesthetics and/or optical properties of an object or component.
The surface properties of many coatings dramatically change on drying, curing and/or aging to become more inert than might be predicted based on the chemistry of their individual components alone. Whilst this phenomenon in part provides the coating with chemical resistance, impact strength and abrasion resistance, it also complicates the process of applying additional coating layers, particularly when they are not applied within a predetermined reapplication window. The same problem arises with applying other entities such as sealants, fillers, stickers and the like, to such coatings. In cases which require the application of additional coating layers and/or other entities, a mechanical or stripping process of the coating is generally necessary before the re-application procedure can take place.
In the specific example of aircraft coatings, it is well known that adhesion will not meet in-service performance requirements when fresh layers of coating are applied over layers which have aged beyond the acceptable reapplication window. The acceptable window may be of the order of days under ambient conditions or hours under conditions of high temperature and/or humidity. Once the reapplication window has been exceeded, the standard practice for applying additional coating layers on aircraft involves mechanical abrasion of the aged coating.
Both chemical stripping and mechanical abrasion have limitations. Mechanical abrasion is labor intensive, the reproducibility is variable, and it is ergonomically costly due to the highly repetitive and vibratory nature of the work. As such there is a pressing need for the development of a surface treatment to improve the adhesion of aged or inert industrial organic coatings towards additional coating layers or other entities, for example, adhesives, sealants, fillers, stickers and the like.
Haack (Surface and Interface Anal, (2000), 29, p 829) investigated the interaction of automotive polyurethane coatings using UV light to generate ozone. Promising results in terms of improved adhesion and reduced water contact angles were produced when paint formulations incorporating TiO2 were subjected to H2O2 and UV light. However, there are obvious practical difficulties associated with this strategy, particularly in terms of its commercial viability for application in areas susceptible to corrosion and for treating larger surfaces. Also the occupational health and safety issues make it less suited to commercial application.
Coating manufacturers have developed a method of improving the procedure of coating stripping through the development of barrier layers which, for example, protect the primer and conversion coating of metal structures from the chemical stripping agents (U.S. Pat. No. 6,217,945). Although this procedure would inevitably improve the rate of paint stripping and reduce the amount of infrastructure down time it still relies on paint removal to provide a surface which will accept a fresh coating layer with acceptable adhesion.
In the biological field, Park et al. (Biomaterials, (1998), 19, p 851) employed the surface urethane NH group to graft chemical species onto polyurethane rubber, whilst Levy et al. (Biomaterials (2001) 22, p 2683) employed a strong base to remove the surface urethane NH proton to accelerate such nucleophilic grafting reactions. Both strategies are unsuitable for activating organic coatings. The chemical reaction kinetics of the first strategy would be too slow to be practical, particularly since, considering the low surface energy and inertness to bonding of such coatings, the urethane NH groups may not be oriented towards the air-coating interface. The use of very strong bases, as per the second strategy, may degrade existing paint layers, resulting in a mechanically weak foundation for fresh coatings to adhere to. Furthermore, the latter strategy is also unacceptable for activating large areas due to corrosion and health and safety considerations.
Other strategies in the biological field have employed free radical techniques to graft molecules onto the surface of biomedical polyurethane surfaces (Matuda et al, J. Biomed. Res., (2002), 59, p 386; Eaton et al, Biomaterials, (1996), 17, p 1977). Although commercially viable, the main difficulty with this strategy lies in promoting actual grafting of the substrate.
Controlled glycolysis or aminolysis as described in Polymer Engineering & Science (1978), 18, p 844, and J. Applied Polymer Science (1994), 51, p 675) has very slow kinetics at room temperature and as such is not a practical solution. The use of reagents such as dimethyl phosphonate (Polymer Degradation and Stability, (2000), 67, p 159) is also not appropriate since they are highly toxic and act too slowly at room temperature.
The strategies disclosed above do not adequately address the need for the development of a surface treatment to improve the adhesion of aged or inert organic coatings to additional coating layers and/or other entities. The problems of commercial viability, health and safety considerations, viable kinetics, applicability to small and large surface areas still remain and need to be resolved.